In VitroStimulation and Inhibition of Adrenocorticotropin Release by Pituitary Cells from Ovine Fetuses and Lambs*

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Abstract
Short term cultured pituitary cells from fetal (63-144 days old) and young lambs (30-120 days old) were tested for their in vitro ability to release immunoreactive ACTH and cAMP in response to stimulation by ovine (o) oCRF1-41, arginine vasopressin (AVP), epinephrine, and forskolin, both in the absence and presence of corticosteroids. For each culture, the percentage of corticotrophs was determined by immunocytochemistry using ACTH antibodies, and the net responses (stimulated-baseline) were expressed per 106 corticotrophs. During gestation, basal ACTH release did not change significantly except at 125 days where it was 2-fold higher than at other fetal stages. Basal ACTH release was 2-fold higher in lambs than in fetuses. In the presence of oCRF1-41, a significant increase in ACTH secretion over basal value was observed at all stages studied. The maximal response decreased from 7.89 .+-. 1.19 ng ACTH 105 corticotrophs-1 3 h-1 at 63 days of gestation to 3.49 .+-. 0.88 ng at 115 days, then remained fairly constant in prepartum animals and lambs. No significant change in the ED50 was observed. The cAMP output induced by oCRF1-41 decreased progressively between 63 and 133 days of gestation from 10.72 .+-. 1.84 to 2.21 .+-. 0.62 pmol, then increased in lambs to values similar to that of 63-day-old fetuses. The ACTH response to AVP was higher than that to oCRF1-41 at 115 days, decreased dramatically in late gestation without modification of the ED50, and remained low in lambs. The ACTH response to epinephrine was always very low. Forskolin-induced ACTH release was lower between 115 and 144 days of gestation than at other stages. The synergistic effect of AVP and epinephrine on both cAMP and ACTH productions stimulated by oCRF1-41 decreased at the end of gestation. The ACTH response to each stimulus was inhibited by dexamethasone, cortisol, and corticosterone (10-9 and 10-7 M) throughout the period studied. This effect was maximum at 63 days of gestation. cAMP release was not altered by glucocorticoids. These results indicate that 1) maximal ACTH response in vitro of corticotrophs to AVP is achieved during fetal life; 2) the capacity of fetal corticotrophs to release ACTH in response to oCRF1-41 is similar to that of lambs throughout the last month of gestation; and 3) corticotrophs are sensitive to the glucucorticoid-negative feedback at least during the second half of fetal life.

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